lib/rsa/rsa-mod-exp.c

// SPDX-License-Identifier: GPL-2.0+
/*
 * Copyright (c) 2013, Google Inc.
 */

#ifndef USE_HOSTCC
#include <common.h>
#include <fdtdec.h>
#include <asm/types.h>
#include <asm/byteorder.h>
#include <linux/errno.h>
#include <asm/types.h>
#include <asm/unaligned.h>
#else
#include "fdt_host.h"
#include "mkimage.h"
#include <fdt_support.h>
#endif
#include <u-boot/rsa.h>
#include <u-boot/rsa-mod-exp.h>

#define UINT64_MULT32(v, multby)  (((uint64_t)(v)) * ((uint32_t)(multby)))

#define get_unaligned_be32(a) fdt32_to_cpu(*(uint32_t *)a)
#define put_unaligned_be32(a, b) (*(uint32_t *)(b) = cpu_to_fdt32(a))

/* Default public exponent for backward compatibility */
#define RSA_DEFAULT_PUBEXP	65537

/**
 * subtract_modulus() - subtract modulus from the given value
 *
 * @key:	Key containing modulus to subtract
 * @num:	Number to subtract modulus from, as little endian word array
 */
static void subtract_modulus(const struct rsa_public_key *key, uint32_t num[])
{
	int64_t acc = 0;
	uint i;

	for (i = 0; i < key->len; i++) {
		acc += (uint64_t)num[i] - key->modulus[i];
		num[i] = (uint32_t)acc;
		acc >>= 32;
	}
}

/**
 * greater_equal_modulus() - check if a value is >= modulus
 *
 * @key:	Key containing modulus to check
 * @num:	Number to check against modulus, as little endian word array
 * @return 0 if num < modulus, 1 if num >= modulus
 */
static int greater_equal_modulus(const struct rsa_public_key *key,
				 uint32_t num[])
{
	int i;

	for (i = (int)key->len - 1; i >= 0; i--) {
		if (num[i] < key->modulus[i])
			return 0;
		if (num[i] > key->modulus[i])
			return 1;
	}

	return 1;  /* equal */
}

/**
 * montgomery_mul_add_step() - Perform montgomery multiply-add step
 *
 * Operation: montgomery result[] += a * b[] / n0inv % modulus
 *
 * @key:	RSA key
 * @result:	Place to put result, as little endian word array
 * @a:		Multiplier
 * @b:		Multiplicand, as little endian word array
 */
static void montgomery_mul_add_step(const struct rsa_public_key *key,
		uint32_t result[], const uint32_t a, const uint32_t b[])
{
	uint64_t acc_a, acc_b;
	uint32_t d0;
	uint i;

	acc_a = (uint64_t)a * b[0] + result[0];
	d0 = (uint32_t)acc_a * key->n0inv;
	acc_b = (uint64_t)d0 * key->modulus[0] + (uint32_t)acc_a;
	for (i = 1; i < key->len; i++) {
		acc_a = (acc_a >> 32) + (uint64_t)a * b[i] + result[i];
		acc_b = (acc_b >> 32) + (uint64_t)d0 * key->modulus[i] +
				(uint32_t)acc_a;
		result[i - 1] = (uint32_t)acc_b;
	}

	acc_a = (acc_a >> 32) + (acc_b >> 32);

	result[i - 1] = (uint32_t)acc_a;

	if (acc_a >> 32)
		subtract_modulus(key, result);
}

/**
 * montgomery_mul() - Perform montgomery mutitply
 *
 * Operation: montgomery result[] = a[] * b[] / n0inv % modulus
 *
 * @key:	RSA key
 * @result:	Place to put result, as little endian word array
 * @a:		Multiplier, as little endian word array
 * @b:		Multiplicand, as little endian word array
 */
static void montgomery_mul(const struct rsa_public_key *key,
		uint32_t result[], uint32_t a[], const uint32_t b[])
{
	uint i;

	for (i = 0; i < key->len; ++i)
		result[i] = 0;
	for (i = 0; i < key->len; ++i)
		montgomery_mul_add_step(key, result, a[i], b);
}

/**
 * num_pub_exponent_bits() - Number of bits in the public exponent
 *
 * @key:	RSA key
 * @num_bits:	Storage for the number of public exponent bits
 */
static int num_public_exponent_bits(const struct rsa_public_key *key,
		int *num_bits)
{
	uint64_t exponent;
	int exponent_bits;
	const uint max_bits = (sizeof(exponent) * 8);

	exponent = key->exponent;
	exponent_bits = 0;

	if (!exponent) {
		*num_bits = exponent_bits;
		return 0;
	}

	for (exponent_bits = 1; exponent_bits < max_bits + 1; ++exponent_bits)
		if (!(exponent >>= 1)) {
			*num_bits = exponent_bits;
			return 0;
		}

	return -EINVAL;
}

/**
 * is_public_exponent_bit_set() - Check if a bit in the public exponent is set
 *
 * @key:	RSA key
 * @pos:	The bit position to check
 */
static int is_public_exponent_bit_set(const struct rsa_public_key *key,
		int pos)
{
	return key->exponent & (1ULL << pos);
}

/**
 * pow_mod() - in-place public exponentiation
 *
 * @key:	RSA key
 * @inout:	Big-endian word array containing value and result
 */
static int pow_mod(const struct rsa_public_key *key, uint32_t *inout)
{
	uint32_t *result, *ptr;
	uint i;
	int j, k;

	/* Sanity check for stack size - key->len is in 32-bit words */
	if (key->len > RSA_MAX_KEY_BITS / 32) {
		debug("RSA key words %u exceeds maximum %d\n", key->len,
		      RSA_MAX_KEY_BITS / 32);
		return -EINVAL;
	}

	uint32_t val[key->len], acc[key->len], tmp[key->len];
	uint32_t a_scaled[key->len];
	result = tmp;  /* Re-use location. */

	/* Convert from big endian byte array to little endian word array. */
	for (i = 0, ptr = inout + key->len - 1; i < key->len; i++, ptr--)
		val[i] = get_unaligned_be32(ptr);

	if (0 != num_public_exponent_bits(key, &k))
		return -EINVAL;

	if (k < 2) {
		debug("Public exponent is too short (%d bits, minimum 2)\n",
		      k);
		return -EINVAL;
	}

	if (!is_public_exponent_bit_set(key, 0)) {
		debug("LSB of RSA public exponent must be set.\n");
		return -EINVAL;
	}

	/* the bit at e[k-1] is 1 by definition, so start with: C := M */
	montgomery_mul(key, acc, val, key->rr); /* acc = a * RR / R mod n */
	/* retain scaled version for intermediate use */
	memcpy(a_scaled, acc, key->len * sizeof(a_scaled[0]));

	for (j = k - 2; j > 0; --j) {
		montgomery_mul(key, tmp, acc, acc); /* tmp = acc^2 / R mod n */

		if (is_public_exponent_bit_set(key, j)) {
			/* acc = tmp * val / R mod n */
			montgomery_mul(key, acc, tmp, a_scaled);
		} else {
			/* e[j] == 0, copy tmp back to acc for next operation */
			memcpy(acc, tmp, key->len * sizeof(acc[0]));
		}
	}

	/* the bit at e[0] is always 1 */
	montgomery_mul(key, tmp, acc, acc); /* tmp = acc^2 / R mod n */
	montgomery_mul(key, acc, tmp, val); /* acc = tmp * a / R mod M */
	memcpy(result, acc, key->len * sizeof(result[0]));

	/* Make sure result < mod; result is at most 1x mod too large. */
	if (greater_equal_modulus(key, result))
		subtract_modulus(key, result);

	/* Convert to bigendian byte array */
	for (i = key->len - 1, ptr = inout; (int)i >= 0; i--, ptr++)
		put_unaligned_be32(result[i], ptr);
	return 0;
}

static void rsa_convert_big_endian(uint32_t *dst, const uint32_t *src, int len)
{
	int i;

	for (i = 0; i < len; i++)
		dst[i] = fdt32_to_cpu(src[len - 1 - i]);
}

int rsa_mod_exp_sw(const uint8_t *sig, uint32_t sig_len,
		struct key_prop *prop, uint8_t *out)
{
	struct rsa_public_key key;
	int ret;

	if (!prop) {
		debug("%s: Skipping invalid prop", __func__);
		return -EBADF;
	}
	key.n0inv = prop->n0inv;
	key.len = prop->num_bits;

	if (!prop->public_exponent)
		key.exponent = RSA_DEFAULT_PUBEXP;
	else
		key.exponent =
			fdt64_to_cpu(*((uint64_t *)(prop->public_exponent)));

	if (!key.len || !prop->modulus || !prop->rr) {
		debug("%s: Missing RSA key info", __func__);
		return -EFAULT;
	}

	/* Sanity check for stack size */
	if (key.len > RSA_MAX_KEY_BITS || key.len < RSA_MIN_KEY_BITS) {
		debug("RSA key bits %u outside allowed range %d..%d\n",
		      key.len, RSA_MIN_KEY_BITS, RSA_MAX_KEY_BITS);
		return -EFAULT;
	}
	key.len /= sizeof(uint32_t) * 8;
	uint32_t key1[key.len], key2[key.len];

	key.modulus = key1;
	key.rr = key2;
	rsa_convert_big_endian(key.modulus, (uint32_t *)prop->modulus, key.len);
	rsa_convert_big_endian(key.rr, (uint32_t *)prop->rr, key.len);
	if (!key.modulus || !key.rr) {
		debug("%s: Out of memory", __func__);
		return -ENOMEM;
	}

	uint32_t buf[sig_len / sizeof(uint32_t)];

	memcpy(buf, sig, sig_len);

	ret = pow_mod(&key, buf);
	if (ret)
		return ret;

	memcpy(out, buf, sig_len);

	return 0;
}